Cast (i.e., molded) refractory containers are used for a variety of purposes. For example, and as further discussed herein, cylindrical, open-ended refractory rings are stacked in order to create an internal volume in which carbon articles are graphitized (e.g., via induction heating). Other types of refractory ceramic containers are employed for various other high temperature applications, including those often termed “refractory crucibles,” which are typically made of various ceramic compositions.
Graphite and graphite composite articles are used for a wide variety of products and components, due in part to their electrical properties as well as their machinability which allows for the fabrication of a wide variety of intricate parts. First, however, the articles formed from carbon and carbon composites must be sintered (graphitized) at high temperatures (up to 2,750° C., or even higher) for long periods (sometimes several days) at various pressures (including a vacuum). Graphitization converts the carbon into a crystalline form, and the required temperature of the graphitization process depends upon the precursor materials and the desired final properties of the graphite articles.
Graphitization is typically performed via induction susceptor heating, often using coreless induction coils. In these processes, heat is transferred indirectly to the carbon articles. For coreless induction coils, refractory rings are stacked inside a water-cooled induction coil. The carbon parts, along with a susceptor (typically made of graphite), are packed inside the refractory rings. The susceptor is then heated via induction, which in turn heats the parts to be graphitized by radiant heat. The induction coil can be quite large (e.g., up to 10 feet in diameter and 15 feet tall, or even larger). The outer diameter of the refractory rings stacked inside the induction coil is smaller than the inside diameter of the induction coil, as the coil typically does not contact the outside of the refractory rings. The rings are sized for both the size of the induction coil as well as the types of articles being sintered.
Older refractory rings often utilized machined graphite rings, since the induction coils were typically very small in diameter (e.g., less than a foot). As induction furnaces increased in size, machined graphite segments were assembled to form rings, sometimes 10 feet in diameter. Machining of graphite is very expensive and time consuming, and wrought with size limitations for monolithic pieces. In addition, the use of segmented refractory rings allows for thermal filler leakage between the assembled bricks, and the graphite would oxidize over time.
More recently, segmented machined graphite components have been replaced with precast SiC rings. Such rings are typically made of a high SiC containing (80% or higher) castable refractory composition, bonded with calcium aluminate cement. However, during use steep thermal gradients from the inside to the outside of the ring typically arise during use, and these thermal gradients often cause fracture in these precast rings. In an effort to prevent such fractures, 1 to 10% by weight of metallic fibers (e.g., ¾″ to 1″, or longer) are added to the refractory castable as crack propagation arrestors. Such fibers are typically made of stainless steel (e.g., 304 SS). Nevertheless, thermal and mechanical stresses imposed by rapid heat-up schedules and higher power induction processes overpower the mechanical deterrent provided by the metallic fibers. In addition, higher metallic fiber content may draw power and couple with the induction field, resulting in power losses and invoking unwanted high thermal expansion in the metallic fibers themselves.
Another type of refractory container in common use are refractory crucibles typically made of various ceramic compositions. Refractory crucibles are used, for example, to melt or otherwise process at very high temperatures, glass, metals, or other substances which require high temperature processing.
By way of further specific example, refractory crucibles are often used to melt glass, and their compositions and shapes have changed little over the past 200 years. See, for example, U.S. Pat. No. 64,558, issued May 7, 1867, incorporated by reference herein. Such crucibles, or refractory pots as they may also be known to those familiar with the art, are manufactured in a variety of shapes and sizes including cylindrical shapes. Various other cross-sectional shapes are also used such as round, elliptical, oval, square (including rounded square), rectangular (including rounded rectangular) or other shapes. Refractory pots typically have a closed bottom and an open top similar to a cup shape. When viewed from the side, in cross-section, the bottom of the refractory pot may be flat, rounded, tapered (e.g., where the bottom meets the interior and/or exterior sidewall of the container), or various combinations of the foregoing.
Some refractory ceramic crucibles are produced as small, monolithic structures the size of a human thumb. These small crucibles are often used, for example, in heating small samples where weight loss is being monitored along with temperature. In large scale production operations, on the other hand, refractory ceramic crucibles can be extremely large—such as ten feet tall (or more) with a four foot feet wide (or more) outer diameter, a wall thickness up to several inches (or more), and weighing several thousand pounds.
Refractory crucibles are produced by any of a number of traditional and non-traditional methods. These include isostatic pressing for small crucibles made of ceramic oxides or silicon carbide (with bonding agents as necessary), jiggering for clay-graphite systems, hand packing clays in the case of traditional glass pots, slip casting ceramic slurries into molds for small laboratory crucibles or casting monolithic refractory castables into pot weighing up to several thousand pounds.
There are generally two types of refractory crucibles: those that are intended to be self-supporting, and those that are not. The latter are typically seated inside another container, such as crucibles which are to be inserted inside an induction furnace with a backup refractory material surrounding it. Dry, vibratable refractory material is typically compacted from a granular form under and around the crucible inside the induction furnace. The material to be melted is then contained and heated within the crucible. An alternate type of crucible is one in which the sidewalls are completely self-supporting. These self-supporting or stand-alone crucibles may be seated on a pedestal or support base and heated from the outer side of the crucible, such as in a gas-fired furnace structure. These crucibles are used, for example, to liquefy the contents of the crucible, such as a glass composition.
Self-supporting refractory crucibles (or pots) such as those used to melt or process zinc, aluminum, copper, various alloys, other metals or glass are typically limited in size. Examples of these types of small-scale, self-supporting refractory containment vessels are made by Morgan Thermal Ceramics and Emhart Glass Manufacturing, Inc. There are a variety of reasons these crucibles are often limited in size. For example, the walls often need to be thin enough to allow heat transfer through the thickness of the wall in order to heat the contents of the crucible, while keeping the operating costs economical. Refractory crucibles having thicker walls take longer to heat, and increased heat input (i.e., increased operating costs). While thinner refractory crucible walls may require less heat input, they also may be more susceptible to premature failure. In addition, the strength of the refractory material at operating temperature may not be sufficient to withstand the head pressure from the weight of the molten liquid and the associated stress from holding the molten liquid contents, such that the crucible wall bows or stretches, and ultimately fails. On the other hand, if the refractory ceramic crucible wall is too thick in these self-supporting designs, the crucible wall becomes susceptible to thermal shock damage due to differential thermal expansion, also creating failure. For example, a glass pot heated from the outside will cause the pot to be hotter at the outer surface than the inner surface, thereby resulting in uneven thermal expansion which can result in stress cracks.
Common failure modes for glass pots and other self-supporting refractory crucibles include the loss of floor and side wall integrity through cracking induced by thermal expansion differentials. In ovoid or square self-supporting, cross-sectional shapes, cracking at a corner wall or a vertical wall often occurs. In some applications, self-supporting crucibles are preheated and then moved into place, thereby creating another thermal shock situation which can result in crack formation and propagation. In other instances, particularly crucibles used in induction furnaces, cracks often arise due to mechanical damage or abuse (e.g., resulting from the lifting mechanisms used to insert the crucible into the furnace, which place bending moments on the sidewalls thereby causing damage), as well as thermal shock. In other instances, mechanical flaws originating during fabrication can also lead to cracking.
Thus, refractory containers such as refractory rings used in induction furnaces as well as refractory crucibles will often fail prematurely for a variety of reasons, including extreme temperatures, wide temperature variations during use, large temperature gradients which lead uneven thermal expansion, mechanical damage, flaws arising during fabrication, and other reasons. While a variety of techniques may exist for preventing or delaying such failures, or otherwise extending the useful life of refractory containers such as the incorporation of reinforcement using metal fibers, it is believed that no one prior to the inventor have made or used an invention as described herein.
The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to the precise arrangements shown.